The present invention relates generally to magnetoresistive (MR) heads in disk drives and more particularly to offset calibration of MR heads.
Digital data storage conventionally utilizes disk drives with rotating rigid disks. Technology improvements have tried to increase the storage capacity and accuracy of disk drives while reducing the weight and power consumption through improvements to each element of the disk drive. In modern disk drives, innovations of anisotropic magnetoresistive (AMR) and giant magnetoresistive (GMR) heads have improved writing and reading of data through the use of separate elements in the recording head as the head flies over a spinning disk. Thus, a writing element writes bits onto the disk, while a reading element reads the bits from the disk by detecting the presence of faint but Emidentifying magnetic fields. The head consists of a thin film inductive write element and an MR read element, i.e., an AMR or GMR sensor between first and second magnetic shields. The read element is typically narrower than the write element to improve the offtrack performance.
In practice, there is an offset between the center of the read and write elements due to longitudinal separation of the elements. When used with a rotary actuator, the head is skewed with respect to the tracks as the actuator moves across the disk. The result is a lateral offset between the read and write head centerlines. Optimum performance is achieved by centering the read head over the data track for read operations and centering the write head over the data track for write operations, which can cause the read head to be partially off-track during a write operation. The geometric offset of the write and read head elements is generally referred to as the MR offset of the MR head. Manifestation of the MR offset is usually seen magnetically and is termed the write-to-read (W/R) offset of the MR head. The terms MR offset and W/R offset are often used interchangeably, and for the purposes of this disclosure, the generally accepted reference of MR offset is used throughout. A large offset is usually not desirable because it requires more time to allow the actuator to center the read element over the data track. This impacts the overall performance of the system.
MR heads may have many forms of instability. One example of instbility is Magnetic Center Shift (MCS), and another example is amplitude instability. The MCS instability usually occurs through one or more stress conditions such as thermal, electrical, magnetic and results in a shift in the W/R offset from the pre-stressed state. The same stress conditions can also engender amplitude instability in which the MR head output signal loses strength.
Voltage and/or current pulses are often used to RESET amplitude instability in MR heads. For example, certain GMR heads with low blocking temperature anti-ferro magnetic materials are resettable through the use of a voltage/current pulse. While the term xe2x80x9cRESET pulsexe2x80x9d refers particularly to this pulse for those GMR heads, it is here generalized to mean any voltage or current that deviates from the intended value. For example, if the intended long term MR bias current is 5 milliamps, then the application of 6 milliamps bias current can be considered to be a xe2x80x98RESET current pulsexe2x80x99. However, resetting can adversely affect the MR offset. For example, in a straight line model for the MR offset, the resetting can result in an MR offset that has an opposite offset and/or slope than before the resetting action. Such a change can cause the servo to move in the wrong direction and lead to data erasures resulting in unrecoverable hard data checks.
Accordingly, what is needed is a system and method for calibrating an MR offset, particularly following an MR reset. The present invention addresses such a need.
The present invention provides method and system aspects for calibrating an MR offset in an MR head. A method aspect includes identifying an occurrence of a calibration condition to initiate an MR offset check, and performing the MR offset check to ensure an MR offset value has not changed beyond an acceptable tolerance in order to maintain proper servoing of the MR head. A system aspect includes a hard disk for storing data, a read/write MR head for reading and writing data to/from the hard disk, and a control unit. The control unit performs an MR offset calibration of an MR offset value for the read/write MR head in response to a calibration condition occurring in order to ensure maintenance of the MR offset value within an acceptable tolerance for proper servoing of the read/write MR head.
Through the present invention, MR offset calibrations are accomplished in a straightforward and efficient manner. These and other advantages of the aspects of the present invention will be more fully understood in conjunction with the following detailed description and accompanying drawings.